Ive been squating for a little while but have never had my form check by anyone other then myself. This is my first time recording myself, I know i need to get lower (trying to get more flexible have tight hip flexores), i also notice that my lower back is arched on my last few reps especially.

Think about this angle and how it would be while squatting. If it's lower than you are squatting, then you have the flexibility, you lack 'stability'. If you can't get those knees very high, the yep, flexibility is your issue.

There are some good articles on squatting flexibility vs. stability on this site.

Also, as people said, definitely warm up. Warm up with whatever you normally use, plus some hip flexor dynamic stretching, as well as some bodyweight and low weight squats.

Focus on getting the TOP of your thigh to at least parallel (lower is better), then you can work on other form points and more weight.